1. Field of the Invention
The present invention relates to an offshore wind turbine generator.
2. Description of Related Art
Offshore wind turbine generators include a monopole type wind turbine that is disposed in a state where the wind turbine floats on a deep ocean as shown in Japanese Translation of PCT International Application, Publication No. 2006-524778.
In this wind turbine, a rotor head on which wind turbine blades are mounted is rotatably mounted on a nacelle. A tower portion that supports the nacelle is not fixed to the sea floor through a base, but the wind turbine is fixed to the sea floor through a mooring cable and floats on a desired sea area.
In such a floating offshore wind turbine generator, since the tower portion is not fixed, the direction of the rotation plane of the wind turbine blades cannot be adjusted in accordance with a wind direction. Therefore, a down-wind type wind turbine that can easily be adjusted in accordance with the wind direction is widely used.
Since it is not possible to adjust the rotation plane in accordance with the wind direction, the nacelle is generally mounted on the tower portion such that it is not rotatable. With this structure, the tower portion autorotates as the orientation of the rotation plane of the wind turbine blades is changed in accordance with the wind direction. Therefore, a portion of the tower portion corresponding to a rotation axis of the wind turbine blade opposite the wind turbine blade is always oriented to the windward.
In the floating offshore wind turbine generator, if the wind turbine blades receive wind, an upper portion of the tower portion attempts to tilt leeward. Thus, great tensile force is applied to the windward side, and great compressive force is applied to the leeward side. Therefore, great stress is generated on the windward side and the leeward side of the tower portion.
According to the patent citation 1, since a structure portion that bears the strength of the tower has a hollow cylindrical structure, the strength thereof is equal in any direction in terms of its cross section. Therefore, a tension leg that applies tension from the outside is disposed on this portion to moderate the unevenly distributed stress.
In case where the tension leg is used, reliability thereof is low because, e.g., the tension leg is stretched so that the stress-moderating effect does not work, or repeated stretching and contraction causes damage on the tension leg due to long-term fatigue. Therefore, the tension leg is frequently replaced by new one. This operation requires, e.g., a large vessel and a large crane, and the operation becomes a large-scale construction.
Since the extra structure, i.e., the tension leg, must be added, production cost is increased, and operation cost is also increased.
The tower portion can be strengthened and the increase in unevenly distributed stress can be suppressed by increasing the diameter of a circle cross section or thickness thereof. In this case, however, the material cost, mechanically machining cost, and transportation cost of the tower portion are increased.